Our work continues to focus on two approaches to relate hematopoietic differentiation and malignant transformation. First, we demonstrated previously that expression of a c-myc or c-myb transgene reversibly blocks terminal differentiation of a mouse erythroleukemia (MEL) cell line. In each case, the DNA binding and transactivation domains are necessary for this block. More recently, we have extended our studies to SCL, Id1, and Id2 as possible proteins that might be involved in the phenotypes of myb or myc transfectants. So far we are unable to determine whether myb or myc block differentiation indirectly by promoting proliferation or directly by disrupting the differentiation pathway. Our long term goal is to understand the molecular mechanisms that are responsible for the apparent inability of hematopoietic tumors to differentiate. Second, we have developed a novel method for subtractive cloning by incorporating polymerase chain reaction (PCR) technology into the preparation and analysis of subtractive cDNA libraries. We have used this method to identify genes that are expressed in most murine plasmacytomas but rarely in B lymphomas. We have identified two classes of genes having this property: 1) genes differentially expressed in plasmacytomas and normal plasma cells but not in B cells; and 2) genes differentially expressed in plasmacytomas but not normal plasma cells (PC). Curiously, a number of genes in the former category are expressed in pre-B and PC but not B cells, suggesting shared functional properties of the cells at either end of the B cell maturation pathway. Recently, we have modified our method in order to prepare a subtractive probe that recognizes genes expressed in two unrelated human myeloma cell lines but neither of two unrelated lymphoblastoid cell lines (LCL). With this probe, we have thus far identified 42 genes that demonstrate this pattern of expression. The finding of 3 adhesion molecules that are differentially expressed in murine and/or human plasmacytomas suggests that one of the key distinguishing features of PCs is their interaction with extracellular matrix and stromal or epithelial cells. Our long term goal is to identify genes that not only mark but also determine the phenotypes of murine and human plasmacytomas and terminally differentiated normal PC.